Semiconductor wafers are usually polished through a plurality of processing steps. Specifically, the processing steps mainly include rough polishing for providing a high flatness to semiconductor wafers and finish polishing for reducing surface roughness. Techniques have been suggested to polish semiconductor wafers with a high accuracy (see, for instance, Patent Literature 1).
A double-side polishing apparatus used for a method according to Patent Literature 1 includes a light emitter that emits a parallel beam and a light receiver that receives the parallel beam, the light emitter and the light receiver being provided on an internal gear in a manner facing each other.
In such a double-side polishing apparatus, when a carrier and a semiconductor wafer held in a carrier hole of the carrier enter a beam transmittable area between the light emitter and the light receiver, the carrier and the semiconductor wafer, which are exposed on upper and lower platens, are irradiated with the parallel beam. The shade of the semiconductor wafer irradiated with the parallel beam is detected by the beam receiver, and a thickness of the shade is directly measured as a thickness of the semiconductor wafer. When the beam receiver detects that the thickness of the shade reaches a target value, the rotation of each of the upper and lower platens is stopped.